Water-containing oil materials of, e.g., an emulsion or aqueous solution type, which are to be diluted with water prior to use, are widely used as flame retardant hydraulic oils or working oils for, e.g., cutting, grinding and rolling. The concentration of oil in the water-containing oil material greatly influences the life of a cutting tool, the roughness of a finished surface, the magnitude of cutting power in the case of cutting, and the life of a grindstone, for example, in the case of grinding. As the oil concentration is increased, cutting performance is generally increased. However, these oil materials are expensive. Thus it is necessary for the oil concentration to be adjusted to a range within which both the performance and oil cost are satisfied.
The concentration of oil in the water-containing oil material varies during its use owing to, e.g., evaporation of water, since the material is usually used while circulating. It is therefore necessary to measure periodically the concentration of oil in the water-containing oil material and to adjust the oil concentration by replenishing water.
Several methods of adjusting the oil concentration have been known. For example, Japanese Patent Publication No. 21366/1980 discloses an oil concentration-adjusting method in which dye is added to the oil material, the oil concentration is determined by measuring the concentration of the dye in the oil material, and based on these measurement results water is added to adjust the oil concentration. Japanese Patent application Laid-Open No. 134098/1982 discloses a method in which a change in the viscosity of the oil material, i.e., a change in the concentration of oil is determined by measuring a change in pressure of the oil material in a pipe, and based on the results thus obtained, water is supplied to adjust the oil concentration.
The former method, however, has disadvantages in that special working should be applied to the pipe and the color of the dye changes with a lapse of time. Also the latter method has a disadvantage in that it cannot be carried out without application of special working to the pipe.